The present invention pertains to the field of telecommunications and relates more especially to an RF or radiofrequency signals amplification block.
1. Field of the Invention
The invention also relates to an RF signals transmission device intended to be arranged at the focus of RF signals focusing means, in particular but not exclusively of the type of a parabola or of a lens of Luneberg type.
2. Related Art
The invention also relates to an antenna/terminal comprising RF signals focusing means at the focus of which is arranged at least one device as cited hereinabove. It relates more especially but not exclusively to such a terminal comprising two devices as cited hereinabove for the tracking of targets, in particular of the nonsynchronous satellites type, just one of these two devices being intended to be active at a given instant.
Hitherto, commercial telecommunications via satellite have been achieved almost entirely via geostationary satellites, which are especially beneficial by virtue of their unchanging relative positions in the sky. However, the geostationary satellite exhibits major drawbacks such as considerable attenuations of the signals transmitted related to the distance separating the user antennas from the geostationary satellite (of the order of 36,000 kilometers, the corresponding losses then rising to around 205 dB in the Ku band) and transmission lags (typically of the order of 250 ms to 280 ms) thus becoming clearly perceivable and perturbing especially for real-time applications such as telephony, video conferencing, etc. Furthermore, the geostationary orbit, situated in the equatorial plane, poses a visibility problem in respect of the regions at high latitudes, the angles of elevation becoming very small for the regions close to the poles.
The alternatives to employing geostationary satellites are:                the use of satellites in inclined elliptical orbits, the satellite then being almost stationary above the region situated at the latitude of its apogee for a duration of possibly up to several hours,        the implementation of constellations of satellites in circular orbits, in particular in low orbit (“Low Earth Orbit” or LEO) or in mid-orbit (“Mid Earth Orbit” or MEO), the satellites of the constellation flying past in turn within visibility of the user terminal for a duration of from some ten minutes to around one hour.        
In both cases, service cannot be provided permanently by a single satellite, continuity of service demanding that several satellites fly over the service area one after another.
An antenna/terminal as defined in the preamble makes it possible to ensure constant communication of the terminal in transmission with the constellation of nonsynchronous satellites.
Such a terminal usually comprises a parabola or a lens of Luneberg type at the focus of which are arranged two sources usually comprising radiating elements, of the type of arrays of “patches”, horns or other radiating elements for transmitting RF signals. These elements are energized by an amplifier block making it possible to supply the source with an amplified signal. Upstream of this amplifier block is a central processing unit making it possible to manage the handovers between two successive low-orbit satellites in the radiation space. It is certain that a single source must be active at a given instant, except during handovers where the problem of the activation of the hitherto inactive source arises.
The role of the amplifier block is to amplify the signal enough to be able to be perceptible by the satellite. It must transmit at a nominal power predetermined by calculation of the satellite/terminal link budget. Specifically, if the transmission is carried out below the nominal power, the satellite recovers a low-level signal whereas if the transmission is performed above the value of the nominal power, the signal conveys distortions which may jam, at the satellite level, the reception of the information contained in the signal (in particular if the signal modulation performed is of the CDMA type).
The initialization of the inactive amplifier block going from the inactive state to the active state is carried out during handover. In the course of handover, the amplifier block must amplify to nominal power, doing so, from the first few milliseconds onwards, at the risk of losing the first few useful data items sent by the following satellite. Now, immediate or quasi-immediate transmission at nominal power during handover is difficult to regulate, due to factors which may cause the gain of the block to vary, such as component spread, variations in power supply, transmission frequencies, temperature, thermal resistance between the junction of a power transistor and the housing containing the amplifier, etc. If the effect due to spread of certain parameters can be reduced by factory calibration, there are others which are more difficult to control such as the junction temperature, making instantaneous regulation of the transmission power almost impossible with the accuracy required by the specifications of the satellite system. Therefore, the amplifier block takes a certain time before amplifying to the predetermined nominal power.
The invention aims to remedy the problems cited above.